The invention relates to a petroleum, gas or geothermal drilling apparatus comprising a central tool and a coaxial bit, wherein the central tool is driven by the rotor of a bottom engine which is fed by the drilling mud and wherein the bit is driven from the surface by the rods of a drill gear which is part of a rotary system, these same rods subjecting the apparatus to an axial load.
An apparatus is known (U.S. Pat. No. 4,862,974) which conforms to this definition; in such an apparatus the aim of the combined action of the two tools is to utilize the energy contained in the mud to augment the total drilling energy, which is necessary for assuring a sufficient forward speed when the diameter of the wells becomes important.
Another possible interest in this arrangement lies in allowing the central tool to turn at an angular speed which is greater than the bit and in this way to obtain linear cutting speeds which are more uniform over the major part of the diameter of the drilled well.
In the drilling apparatus described in the patent (U.S. Pat. No. 4,862,974), the drilling apparatus completely joins together the base of the bit and the stator of the bottom engine.
Under these circumstances, power distribution is optimal only in the single case where the drilled ground and the cutting conditions to which both tools are subjected are such that the forward speeds which are made by both are identical when they are separately driven.
Any deviation in respect to this ideal situation manifests itself in a mediocre instantaneous power distribution and a problematical achievement of the hoped-for advantages.
One of the results envisioned by the invention is to remedy this disadvantage.